Along with all the other arguments against using body mass index (BMI) as a way to measure someone’s weight status as overweight or obese, a new study suggests that it may be harmful for postmenopausal women.
The study, which was published in the March 2018 edition of the journal Menopause, looked at more than 1,300 postmenopausal women ages 53 to 85 and measured their BMI and body fat percentage. A woman was considered obese if she had a BMI greater than 30 and more than 35 percent body fat.
The researchers found that “using a BMI of 30 to define obesity in postmenopausal women results in misclassification,” says the study coauthor Hailey Banack, PhD, a postdoctoral associate in the department of epidemiology and environmental health at the University of Buffalo in Buffalo, New York. If obesity is classified as 35 percent body fat, a BMI of 30 will correctly flag only 55 percent of women as obese.
That may be a problem. “This type of BMI misclassification could result in an underestimate of true disease risk,” says Dr. Banack. Clinicians who rely on BMI as a health marker may be missing women who would benefit from an intervention to lose weight. More body fat — particularly visceral fat around the midsection — is associated with a greater risk of heart disease, diabetes, and osteoarthritis, among other problems, according to Harvard Medical School.
That said, as Banack notes, the idea behind the research isn’t to advocate lowering the BMI range indicating obesity. “The question of which cut-point should be used is still unanswered,” she says. “We don’t suggest that having women target a specific BMI cut-point should replace clinicians encouraging women to maintain a healthy waist circumference or living a healthy lifestyle,” she adds.
Why Postmenopausal Women Shouldn’t Stress Too Much About BMI
Indeed, the number may not be something you should give too much credence to. “BMI was developed as a one-size-fits-all solution to quantifying body composition, but it may not be applicable for postmenopausal women,” says JoAnn Pinkerton, MD, the executive director of the North American Menopause Society, who was not involved in the research.
That’s because women go through changes in their body composition as they age. After menopause, for example, women lose bone and muscle mass. “The same BMI in a 65-year-old woman may be based more on visceral fat and less on bone and muscle mass,” she says. Visceral fat increases the risk of chronic disease, so even if a 65-year-old woman weighed the same when she was 50, she’d be at a higher risk for heart disease now.
The issue? Doctors may not be finding the women who are at an elevated risk for disease. “If women with more abdominal fat mass are classified as normal weight using only BMI instead of measuring waist circumference or using CT scan or MRI to calculate percentage of fat, those women could be falsely reassured and miss the chance to reduce the risk of cardiovascular disease or diabetes that are obesity-related health concerns,” says Dr. Pinkerton.
Still, many obesity experts advocate against the use of BMI. “In a clinical setting, you shouldn’t use BMI as a measure of anything,” says the weight loss specialist Charlie Seltzer, MD, who is in private practice in Philadelphia. That said, all the preventive measures and necessary testing should be done for everyone, including postmenopausal women, regardless of her BMI, he says.